A slight energy depletion occurs along the way

The answer is simple. When light is propagated over very long distances in the range of hundreds of millions of light years, a slight energy depletion occurs along the way. The photon package increases in size by the energy depletion. The purple light thereby becomes blue light, it becomes a larger package, and the blue light becomes green light, and the green light becomes yellow light, and so on. The entire spectrum of the light becomes shifted towards the red, and the red, of course, gets shifted off the visible spectrum.

Now, the red shift phenomenon makes sense. It has nothing to do with light from distant galaxies being stretched by a moving light source that is racing away from us. The entire Big Bang theory, thereby falls apart.

Yes, the wavelength of light appears to be stretched thereby, but this is the result of the changing size of photons that results from gradual energy depletion.

The amount of the red-shift is typically a factor of the distance that light has to cross from far-away galaxies before it arrives at our door.